As the number of services provided by wireless mobile communication devices increases dramatically, so does the need for mobile communication devices that can handle the various forms of signal formats required to provide the mobile communication services. For example, devices in cellular telephones may need to adhere to standards such as a Global Systems for Mobile communication (GSM) standard, a Personal Communication Services (PCS) standard, an EDGE standard, and a Digital Cellular System (DCS) standard. The standards all require precise output power control over a large dynamic range in order to prevent channel interference.
The key component common to mobile communication devices is a power amplification device. Before reaching the power amplification device, a radio frequency (RF) transmission signal is too weak for transmission to a cellular base station. Therefore, it is the function of the power amplification device to boost the power of the RF transmission signal. However, the power amplification device needs to amplify the RF transmission signal such that the RF transmission signal complies with a specification known as a “burst mask.” The burst mask provides requirements for the mean power of the RF transmission signal transmitted in a particular timeslot. More specifically, the burst mask specifies an allowable ramp-up period, duration, and ramp-down period of the mean power of the RF transmission signal during a timeslot. Each of these timeslots may have different burst mask specifications and the RF transmission signal needs to comply with each of these burst mask specifications to prevent switching spectrum interference in other timeslots.
FIG. 1 illustrates a prior art power amplification device 10 that includes a power amplification circuit 12. The power amplification circuit 12 includes multiple power amplifier stages, 14, 16, and 18. To provide sufficient power for transmission of the RF transmission signal 19 by an antenna, each power amplifier stage may provide amplification in a cascaded manner according with an amplification gain provided by each power amplifier stage, 14, 16, and 18. Generally, the power amplifier stage 18 is referred to as the final amplifier stage and the amplifier stages 14 and 16 in combination are referred to as a driver amplifier stage.
To meet the burst mask specifications, the amount of power being delivered by the power amplification circuit 10 needs to be appropriately controlled. Thus, a voltage regulation circuit, such as a low-drop-out (LDO) circuit may be utilized to provide a regulated voltage that powers the amplification of the RF transmission signal 19 by the power amplification circuit 12. The driver amplifier stage of the power amplification circuit 12 may designed to provide a maximum output power having a significant margin relative to the rated output power to account for fabrication variations, temperature variations, and/or the like. Nevertheless, if a single voltage regulation circuit is used to provide the same regulated voltage to each of the power amplifier stage, 14, 16, 18, excessive currents are produced in the driver amplifier stage during typical operating conditions which comes at a cost to power efficiency.
Referring now to FIGS. 1 and 2A, to improve the power efficiency, the prior art power amplification device 10 uses a first voltage regulation circuit 20, and a second voltage regulation circuit 22. Each of the voltage regulation circuits 20, 22 generates a regulated voltage VREG1 and VREG2, respectively from a supply voltage VS. To control the regulated voltage levels of the regulated voltages VREG1 and VREG2 and therefore the amplification gain of the amplifier stages 14, 16, 18, both of the voltage regulation circuits 20, 22 receive a voltage control signal VRAMP. A graph of the regulated voltages VREG1 and VREG2 as a function of a voltage control signal VRAMP is shown in FIG. 2A. In order to reduce current consumption, the voltage adjustment gain of the first voltage regulation circuit 20 needs to be very low. Unfortunately, this results in a reduction of the maximum output power because the regulated voltage level of the regulated voltage VREG1 is never allowed to rail near a supply voltage level of the supply voltage, VS.
Referring now to FIGS. 1 and 2B, FIG. 2B demonstrates another disadvantage of the prior art power amplification device 10. FIG. 2B is a graph of the regulated voltages VREG1 and VREG2 versus the phase angle when the load Voltage Wave Standing Ratio (VWSR) is held constant. As shown in FIG. 3, the regulated voltage VREG1 varies with the phase angle to maintain a constant output power. Nevertheless, once the regulated voltage VREG2 rails there is no more available power. On the other hand, the regulated voltage VREG1 remains constant regardless of phase angle thereby indicating an inability of the driver amplifier stage to provide additional output power and increased current consumption by the driver amplifier stage when operating into a mismatch.
Therefore, what needed are power amplification devices with voltage regulation circuits that can better preserve the maximum output power of the power amplification device while providing increased power efficiency and better performance during mismatch conditions.